Quantum communication networks are high on Europe's agenda, with particular focus being given to quantum memory or information storage. Meeting the challenge head on to make such information accessible to users is a team of scientists from Denmark who used two 'entangled' light beams to store quantum information. The research study, supported by the EU with a combined funding of almost EUR 16 million, is presented in the journal Nature Physics.

Experts believe quantum networks will offer users improved information security compared to what is currently available. One of the important components of quantum communication is entanglement between quantum systems including two light beams. In a nutshell, entanglement is the connection of two light beams. These light beams have well-defined common characteristics like common knowledge. Under the laws of quantum mechanics, a quantum state can be used to transfer information securely and copy-free.

Led by Professor Eugene Polzik, the scientists from the Niels Bohr Institute at the University of Copenhagen successfully stored two entangled light beams in two quantum memories. They used a forest of mirrors and optical elements like beam splitters and wave plates on a large table, resulting in light being sent on a labyrinth journey over 10 metres long. Using the optical elements helped the researchers, who are part of the institute's Quantop group, to control the light and regulate the size and intensity, effectively ensuring that the light's wavelength and polarisation meet the experiment's needs.

The researchers explained that they created the entangled light beams by sending a single blue light beam through a crystal where the blue light beam is split up into two red light beams. The two red light beams have a common quantum state because they are entangled. According to the team, the quantum state itself is information.

The labyrinth of mirrors and optical elements receive the two light beams that then reach the two memories. For the purposes of this study, the scientists used two glass containers filled with a gas of caesium atoms. The team said the atoms' quantum state contains information in the form of a so-called spin, which either can be 'up' or 'down'.

The researchers can then compare it with computer data that consists of the digits 0 and 1. The quantum state is transferred from the two light beams to the two memories after the light beams pass the atoms. The end result? The information is stored as the new quantum state in the atoms.

'For the first time such a memory has been demonstrated with a very high degree of reliability. In fact, it is so good that it is impossible to obtain with conventional memory for light that is used in, for example, internet communication. This result means that a quantum network is one step closer to being a reality,' Professor Polzik said.

The research study was funded in part by six EU projects: Q-ESSENCE, HIDEAS, CORNER, COMPAS, COQUIT and EMALI.

The EMALI ('Engineering, manipulation and characterisation of quantum states of matter and light') project secured EUR 4.39 million under the Marie Curie Research Training Networks budget line of the EU's Sixth Framework Programme (FP6).